1. Field of the Invention
The present invention relates to optical transmission systems, and more particularly, to an optical transmission system having linear repeater sections that perform linear amplification of optical signals.
2. Description of Related Art
The advancements in optical amplification techniques using optical fiber amplifiers have enabled development of linear optical repeaters which optically amplify attenuated incoming signals and retransmit the proportionally enhanced signals to the next part of the transmission line. Optical communications systems employing such optical repeater techniques are now at the commercial stage, where the communication line is composed of a plurality of linear repeater sections each including linear optical repeaters to allow a long-distance data transmission.
FIG. 25 is a diagram showing a typical configuration of an optical communications system that uses conventional linear optical repeaters. In FIG. 25, regenerative intermediate repeaters (RST-REPs) 204, 207, and 210 are deployed between two terminal multiplex repeaters (LT-MUXs) 201 and 213, where the linear optical repeaters mentioned above are used in each section of the transmission line divided by those major repeater systems. Such sections are referred to as "linear repeater sections," each involving a predetermined number (N.sub.LIM) of linear optical repeaters (L-REPs). More specifically, a first linear repeater section between the LT-MUX 201 and RST-REP 204 contains L-REPs 202-203, and similarly, the other three sections contain L-REPs 205-206, 208-209, and 211-212, respectively. In each linear repeater section, the cascaded L-REPs will repeatedly amplify the optical signal to overcome the effects of signal attenuation during its travel over the optical transmission line. However, this repeated linear amplification will also cause some deterioration in the signal-to-noise ratio (S/N) due to the optical noises introduced and accumulated in the amplification processes. Also, the transmission characteristics will be degraded due to the wavelength dispersion of optical signals. It is, therefore, necessary to regenerate or refresh the signal at every point where such signal quality degradation reaches a predetermined limit. The aforementioned constant N.sub.LIM is a critical number that is predetermined for this purpose.
The RST-REPs 204, 207, and 210 reform the optical signal waveform by what are collectively called "3R" (i.e., Reshaping, Retiming, Regenerating), and carry out termination and insertion of the Regenerator Section Over Head (RSOH) according to the Synchronous Digital Hierarchy (SDH) standards. That is, the optical transmission system of FIG. 25 is based on the SDH signal transmission scheme, where control signals are conveyed in a specific bit field called "Section Overhead (SOH)" defined as part of the SDH frame format. The RSOH is included in the SOH for use in the interactions between two repeaters or between a repeater and a terminal system. The RSOH insertion and termination processes involve several functions such as frame alignment synchronization, error monitoring, and alarm status communication.
While the optical signal regeneration is crucial at both end points of each linear repeater section, the RSOH insertion and termination are not necessarily required at every such point. However, the conventional optical transmission system shown in FIG. 25 deploys the regenerative intermediate repeaters (RST-REPs) 204, 207, and 210, in which RSOH insertion and termination functions are implemented in vain.
In the conventional optical communications system, an increased noise factor NF or any other kind of gradual performance degradation that happened to the L-REPs 202-203, 205-206, 208-209, and 211-212 would cause some bit errors (i.e., recognizing a signal "1" for "0" or vise versa) at a later receiving stage, because of deterioration in S/N ratios. To overcome the possible bit errors, some redundancy design or fault tolerant design must be introduced into the optical transmission system.
Although it may not be possible to implement fully duplexed configuration in the RST-REPs 204, 207, and 210 when some tight cost constraints are present, the demands for the more reliable regenerative repeaters still exist. To meet those demands at a low cost, it is necessary to devise some practical ways to introduce functional redundancy into the signal regeneration units that provide the signal regeneration functions.